In the terminology of traditional printing the term trapping (more particularly wet trapping and dry trapping) is believed to have been applied first to the press technique of overprinting on a previously printed page. Such a technique has been widely used to produce color prints by printing different color impressions in succession on the same sheet. Print technologies that layer multiple colorants in this way are sensitive to mechanical positioning errors. The different color layers must be almost perfectly aligned, or registered, over the whole sheet for good results. A relative lateral misalignment even as small as a fraction of a pixel in a digital system, or its equivalent in traditional printing, will usually be visible as undesired gaps or overlaps at the edges of printed shapes where colors change and involve different layers. Gaps that reveal a white or light-colored substrate are the most visible and objectionable. Overlaps are also visible, but not nearly as objectionable. Because sufficiently precise alignment is seldom practical, various pre-press techniques have been developed to close the gaps by creating at the pre-press stage small overlaps between abutting colors to mask registration problems in the printing process itself. The essentials of the pre-press technique are known as spreading and choking and collectively called trapping. In view of the potential ambiguity, the subject of this invention is trapping in the pre-press context in order to reduce the visibility of misregistration artifacts.
The best visual forms for changes to the image to be printed have been learned over the years, mostly in the traditional printing industry, and codified into a set of rules. In traditional printing, implementation of the rules required knowledge and craftsmanship, but because they are systematic and thorough they have proved suitable for automatic implementation in contemporary digital printing systems. However, they are not simple, they do not completely address all practical cases and their interaction with other digital image path processes presents particular difficulties further compounded by the challenges of inexpensive and efficient implementation.
FIG. 1 is a general diagram of a digital printing system consisting of a personal computer 1a that can communicate with a color printing device comprised of a device controller 1b and a color marking mechanism 1c. The device controller 1b accepts document data to be printed, delivers raster image data to the color marking mechanism and provides control functions. The color marking mechanism prints the raster images on plain paper or other suitable physical media. Imperfections in the mechanism may give rise to color registration artifacts which can be ameliorated by trapping.